The preparation of semiconductor devices made from silicon, gallium-arsenide, and the like, begins with growing a monocrystalline boule. The boule is then sliced into thin disks called wafers. The wafers are then circumferentially ground, lapped, chemically etched, polished, and cleaned.
The lapping step is conventionally carried out in large orbital lapping machines that are well known in the art.
In the lapping process, the wafers are placed in circular plates called carriers. The carriers have holes prepared in them that are slightly larger in diameter than the wafer to be lapped, and are slightly thinner than the target thickness of the wafers at the end of the lap cycle. The carriers are circular shaped, and usually have gear teeth around the outside periphery. The gear teeth interact with a center gear and an annular gear such that a drive motor turning the gears causes the carrier to rotate around the center gear and around itself in an orbital motion. The wafer-containing holes in the carriers are placed toward the outer edges of the carrier such that they rotate around the center of the carrier itself during machine operation. By changing the gear ratios of the carriers, annular gear, and center gear, the direction of rotation of the carriers can be controlled and changed as needed to control flatness and wear of lapping plates.
Both the carriers, and the wafers contained within them, are supported by a bottom lapping plate. Upon starting a lap cycle, a top lapping plate lowers onto the wafers. A slurry containing an abrasive and other components such as a soap, a rust inhibitor, or a surfactant as desired, is introduced to the wafers through slurry supply holes in the top plate. One or both of the plates are typically linked to either the center gear or the annular gear such that they rotate in a controlled ratio and direction along with the center gear, annular gear, and carrier. The combined rotation of each of these items results in the wafers being moved in a circular motion within the carrier, around the carrier, and around the sun gear, with the top and bottom lapping plates rubbing against the two flat surfaces of the wafer. Wafers lapped in such a manner have very smooth and flat surfaces, with a high degree of uniformity between wafers lapped in the same cycle.
As the lap cycle continues, a force is exerted on the wafers from the top plate either from the weight of the top plate or from a mechanical means. The force exerted on the wafers, the motion of the wafers around the plates, and the abrasive in the slurry combine to lap away the surfaces of the wafer in small increments. When the desired thickness of the wafers is reached, the lap cycle is stopped, and the wafers are removed from the lap machine.
The lapping plates used for the process are typically metal, and have grooves cut into the surfaces that are in contact with the wafers. Among other functions, the grooves assist in supplying slurry to the entire surface of the wafers, carriers, and plates, and to facilitate the removal of the used slurry and residue of the lapped wafer. The grooves can be cut into the plates in many different patterns as the user desires. The grooves are typically about 5 millimeters wide, and initially about 15 millimeters deep. Over time however, the surfaces of the lapping plates are abraded away, and the grooves become much more shallow. As a result, the lapping plates either need to be replaces, or resurfaced with new grooves cut into the surfaces.
As the lap machine is used in repeated cycles, the grooves become clogged with the residue slurry and the material that has been lapped away from the wafers, plates, and carriers, inhibiting removal of the waste slurry and becoming a trap for particles and contaminants. The grooves, when clogged, create a buildup of slurry on the bottom plate that causes the wafers to float during the start of the lap cycle. Wafers then can leave the confines of the wafer carrier, and break in the lap machine. Shards of the wafer then get lodged in the residue slurry in the grooves of the bottom plate and become projectiles that can scratch wafers lapped in future cycles.
There is therefore a need of an apparatus to clean the residual slurry out of the grooves of the plates of lap machines.